Muscle fiber type effects on energetically optimal cadences in cycling
Introduction
Differences in the mechanical and energetic properties of isolated mammalian slow-twitch (ST) and fast-twitch (FT) muscles, motor units, and muscle fibers have been well documented (Bottinelli and Reggiani, 2000; Burke, 1994; Rall, 1985). In general, ST muscles are slower, less powerful, and more economical at force generation than FT muscles from the same species. Furthermore, peak efficiency of ST muscle fibers occurs at slower shortening speeds than in FT fibers (Barclay, 1994; Bottinelli and Reggiani, 2000). In humans, the limb muscles are heterogeneous with respect to fiber type distribution (Johnson et al., 1973), but mechanical and energetic descriptors of muscular performance seem to scale quite predictably with the proportions of FT and ST muscle fibers (Coyle et al., 1979; Coyle et al., 1992; Thorstensson et al., 1976).
Bicycle pedaling has been used as an experimental paradigm in numerous investigations of fiber type-related issues, and a few of these studies particularly exemplify how muscle fiber type composition may affect energy expenditure and the energetically optimal cadence during pedaling (Coyle et al., 1992; Hansen et al., 2002; Hintzy et al., 1999). Coyle et al. (1992) found the percentage of ST muscle fibers in vastus lateralis to be positively correlated with gross () and delta () efficiency in endurance cyclists, and a similar relationship () for gross efficiency was reported by Hansen and colleagues (2002). The four subjects in Coyle et al. (1992) with the highest percentage of FT muscle fibers (63% FT) expended 14% more metabolic energy than the four subjects with the lowest percentage of FT fibers (27% FT), at the same mechanical power output and cadence. Supporting evidence suggested that the between-subject differences in efficiency could not be explained by other factors such as pedaling technique (Coyle et al., 1991), and similar results were found in a different motor tasks (Coyle et al., 1992).
More recently, Hintzy et al. (1999) reported a positive relationship () between the energetically optimal cadence and the cadence at which short-term power output was maximized (maximal power cadence) in three groups of trained noncyclists. In addition, the energetically optimal cadence was significantly higher in the group of explosively trained athletes (60.8 rev min−1) than in the endurance-trained group (54.0 rev min−1). Hintzy and colleagues did not assess muscle fiber type directly, however, subjects with higher maximal power cadences tend to have a greater proportion of FT muscle fibers (Hansen et al., 2002; Hautier et al., 1996), suggesting a positive relationship between energetically optimal cadence and the percentage of FT fibers. Thus, the differences in submaximal energy expenditure between subjects, and the cadences at which energy is minimized can presumably be explained largely by muscle fiber type distribution.
These findings are notable in that they point to the role of fundamental muscle properties in determining the energetics of pedaling, rather than experience or training history. In contrast, others have suggested that during training cyclists adapt to become more efficient at pedaling, often with the adoption of a relatively high cadence (Coast and Welch, 1985; Hagberg et al., 1981). While muscle fiber type distribution may influence energy output and optimal cadence during pedaling, it is difficult to determine the direct influence of fiber type distribution per se. Quantification of fiber type is usually based on a small number of biopsies, typically obtained from just one muscle. An alternative approach to study the effects of fiber type distribution on pedaling energetics is through musculoskeletal modeling and computer simulation. Modeling and simulation are common tools for investigating the mechanics and control of pedaling (Neptune and Hull, 1999; Raasch et al., 1997; van Soest and Casius, 2000), and the use of a model of muscle heat production (Umberger et al., 2003) further allows the energetics of pedaling to be investigated. Computer simulation can be an especially powerful tool, as it provides estimates of many important quantities not accessible in an experimental setting (e.g., muscle forces, muscle energy expenditure).
The objective of the present study was to determine the effects of muscle fiber type distribution on muscle and whole-body energetics during submaximal pedaling using a modeling and simulation approach. This objective was met using two musculoskeletal models that reflected a realistic range of fiber types for human lower limb muscles. The specific quantities investigated were the cadences at which submaximal energy expenditure was minimized, as well as the mechanical and energetic output of the lower limb muscles across a range of cadences. The only difference between the two models was muscle fiber type distribution. Thus, it allowed for an assessment of the relative effect of this factor on submaximal pedaling performance.
Section snippets
Methods
Two models of human bicycle pedaling were developed that were identical in every way except for the mechanical and energetic properties of the muscles. One model had muscle parameter values that were consistent with a high percentage of FT fibers (FT model), while the other model reflected a low percentage of FT muscle fibers (ST model). Muscle energy expenditure was estimated using a model in which energy liberation depended on muscle fiber type distribution.
Results
After optimization of the muscle excitation parameters, both the FT and ST models were able to generate pedaling at the target power and cadences (within ±1%) while reproducing the major kinematic and kinetic features of human cycling (Fig. 1). Root mean square errors between simulation and experimental data ranged from 0.63° to 2.58° for pedal angle and 0.99 to 4.66 N m for crank torque. Timings of muscle activity bursts, based on the optimized excitation profiles, were also in reasonable
Discussion
The simulations demonstrated that varying muscle fiber type distribution changes energetically optimal pedaling rate in a manner consistent with differences observed in the literature. The energetically optimal cadences (FT: 64 rev min−1, ST: 55 rev min−1) were comparable to experimental values (Coast and Welch, 1985; Marsh and Martin, 1997; Seabury et al., 1977), and the 9 rev min−1 difference between FT and ST models was similar to the 7 rev min−1 deviation reported by Hintzy et al. (1999). While
Conclusion
The present simulation results supported the contention that between-group variations in muscle fiber type distribution contribute significantly to differences in submaximal energetically optimal cadences (Hintzy et al., 1999). However, the whole-body rate of energy expenditure during cycling does not appear to be a valid indicator of the metabolic demands placed on the lower limb muscles across cadences. Mechanical efficiency, which depends on the mechanical and energetic properties of the
Acknowledgements
This research was completed at the Exercise and Sport Research Institute, Department of Kinesiology, Arizona State University in Tempe, AZ. Financial support was provided in part by a National Science Foundation IGERT Grant (DGE-9987619) titled “Musculoskeletal and Neural Adaptations in Form and Function”.
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